Polish-Soviet relations went through three periods during the second world war. Between 1939 and 1941, the nazi-Soviet allies destroyed the Polish state while the Soviets extinguished Polish influence in the Eastern Territories which they had occupied in September 1939. After April 1943 Stalin marginalized the â€˜Londonâ€™ Poles and asserted his favoured eastern frontier while his pro-Soviet forces took over Poland. In the middle period of formal Polish-Soviet relations between the nazi invasion of the USSR in June 1941 and Stalinâ€™s breaking-off of diplomatic relations as a result of the German revelation of Katyn in April 1943, Sikorskiâ€™s government-in-exile and the Western Allies attempted to establish a genuine London Polish-Soviet understanding. Post-communist documentation now allows a full appreciation of the role played by Stalinâ€™s massacre of 14,700 Polish PoWs (aka Katyn), mainly officers and policemen, held in three NKVD-run special camps and executed in three separate execution locales in April-May 1940. Admitting responsibility for the massacre was not an option for Stalin as it would have weakened his regimeâ€™s legitimacy and bargaining position and alerted Western Ã©lites and public opinion to its totalitarian character. Why the Western Allies went along with this remains highly controversial, as it involves revised judgments about both the morality and effectiveness of their warfighting strategy during the second world war and its consequences for the postwar settlement.